feat: Add ProgramWriter utility struct for plugin communication

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

src/plugin.rs

@@ -0,0 +1,25 @@
+use std::io::Write;
+
+use derive_more::{Deref, DerefMut};
+
+/// A wrapper around a child process's stdin that implements the Write trait.
+///
+/// This struct allows writing data to an external process's standard input
+/// in a way that's compatible with Rust's I/O traits.
+#[derive(Deref, DerefMut)]
+pub struct ProgramWriter {
+    /// The stdin handle of a spawned child process
+    #[deref]
+    #[deref_mut]
+    pub stdin: std::process::ChildStdin,
+}
+
+impl Write for ProgramWriter {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.stdin.write(buf)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.stdin.flush()
+    }
+}